RCS E2E and Identity: Mapping Phone Numbers to Verified Recipients Without Breaking Privacy

Hook: Why linking RCS-capable numbers to verified recipients is now a business risk and an operational opportunity

Delivering sensitive content at scale means two competing demands: confirming the recipient and minimizing exposure of personally identifiable information (PII). For technology teams in 2026, that challenge is acute: RCS is maturing into an encrypted, feature-rich channel while regulators, fraudsters, and users demand tighter privacy and explicit consent. This article maps practical architectures, code patterns, and compliance controls you can use to link an RCS-capable phone number to a verified recipient identity without breaking privacy.

Executive summary — what to build first (inverted pyramid)

Build three integrated layers: verification & attestation (confirm device/control), privacy-preserving linkage (tokens, hashes, PSI), and consent & audit (recorded opt-in and revocation). Prioritize ephemeral or irreversible identifiers for linkage, use attestation sources where available (carrier or identity providers), and log consent with cryptographically signed tokens or JWTs. These steps lower PII exposure, improve deliverability for RCS (versus SMS), and create auditable trails for compliance teams.

Context in 2026: Why RCS changes the rules

Two developments through late 2025 and early 2026 are reshaping design decisions:

• GSMA and ecosystem upgrades to RCS (Universal Profile updates) have expanded feature parity with OTT channels and made E2EE a central specification target.
• Major vendors began shipping end-to-end encryption for RCS-capable conversations — Apple signalled movement in iOS 26 betas, and Android clients continued to improve MLS-based schemes. That shift makes it feasible to send richer, private content without relying on OTP links or email fallbacks.

Combined, these trends mean you can rely on RCS for secure delivery, but you still need robust identity mapping and consent models that don’t require storing raw phone numbers in plaintext.

Key tradeoffs: SMS vs RCS for verified recipient workflows

• SMS — universal, but plaintext and increasingly unreliable for high-risk workflows; OTPs are easy to phish and spoof.
• RCS — richer, supports delivery/read receipts and E2EE (growing support), and reduces friction for authentication (in-app verification flows), but requires mapping numbers to keys/identities while protecting PII.

Design principles for privacy-preserving phone->recipient linkage

1. Minimize PII retention — store irreversible digests or tokens instead of raw phone numbers where possible.
2. Use cryptographic salting & key management — compute HMACs with a server-held secret in an HSM/KMS; rotate keys with re-hash plans.
3. Prefer attestation over raw possession — get carrier or device-based attestations (where available) to assert control without storing more identifiers.
4. Make tokens ephemeral and auditable — short-lived tokens reduce risk, and signed consent artifacts create an audit trail.
5. Apply PII segmentation — separate systems that need to reach the recipient (messaging) from those that store identity attributes (KYC, profile).

Practical architectures — three patterns you can implement today

1) Deterministic HMAC mapping (simple, auditable)

Store only an HMAC of the phone number for everyday matching. Use a server-side secret stored in an HSM/KMS. This supports fast lookups and is irreversible to anyone who lacks the secret.

Flow:

1. User enters phone number in your UI (or you receive a recipient list for ingestion).
2. Immediately compute phone_hash = HMAC-SHA256(kms_key, E.164_number) and store phone_hash → recipient_id mapping.
3. Use that hash for deduplication and matching; keep raw numbers confined to an ingest service with limited retention.

// Node.js example: compute a phone hash (concept)
const crypto = require('crypto');
function phoneHash(phoneE164, secret) {
  return crypto.createHmac('sha256', secret).update(phoneE164).digest('base64url');
}
  

Implementation notes: rotate the secret periodically and keep a key-identifier in the DB so you can re-hash if needed. If you must recover plain numbers for legal reasons, keep a tightly controlled unencrypted vault with multi-person approval and logging.

2) Tokenization with one-way tokens and ephemeral association

Issue a short-lived token that binds a phone to a recipient when consent is confirmed. This is helpful for campaign delivery or time-bounded content access.

1. Verify control with an RCS verification message or OTP fallback.
2. After verification, issue a signed JWT token: {sub: recipient_id, ph_hash: phone_hash, att: source, exp: <short> }.
3. Use the token as a delivery key; the messaging system holds only the token and the hash, not the raw number.

// JWT payload example (conceptual)
{
  "iss": "https://api.yourorg.com",
  "sub": "recipient:12345",
  "ph": "uE6...base64url",    // phone_hash
  "att": "carrier_attestation", // optional
  "consent": {
    "type": "rcs_agreement",
    "ts": "2026-01-10T12:00:00Z"
  },
  "exp": 1716000000
}
  

Tokens make it easier to revoke access (blacklist specific JWT jti values) and to support consent revocation without touching the canonical identity store immediately.

3) Privacy-preserving list hygiene with Private Set Intersection (PSI)

When you need to match large recipient lists with partner data (e.g., blocked numbers, suppression lists, or third-party verification datasets) without sharing raw numbers, use PSI.

• PSI lets two parties compute intersection size or exact matches using cryptographic protocols without revealing non-matching elements.
• Use it to check for fraud signals, known-bad numbers, or consent status held by a partner without exposing your entire list.

Vendor implementations and open-source libraries matured in 2024–2026, and they are now production-ready for lists in the millions when orchestrated with batching and parallelization.

Attestation sources — what to trust and how to capture it

Attestation proves that the device or number is controlled by the claimed entity. Sources include:

• Carrier attestations — the strongest source for phone number control; accessible via MNO APIs in some markets or through partnerships with aggregators.
• Device-based attestations — Android SafetyNet / Play Integrity and platform-level key attestation for apps, or app-signed tokens for in-app verification. See developer onboarding guidance for how to capture and verify these artifacts.
• Third-party identity providers — OAuth-centric IdPs or banking-grade ID verification services that can assert identity without returning phone numbers.

Design: capture attestation metadata (attestation source, timestamp, evidence_url or signature) and store it paired with the privacy-preserving token/hash so you don’t need raw PII for later audits.

Consent capture and audit trail patterns

Consent must be granular, explicit, and auditable. For RCS flows you can embed consent capture directly in the conversation (when E2EE is available) and sign the consent artifacts.

• Present selectable purposes and retention windows at initial verification. Record the exact payload the user consented to and sign it server-side.
• Issue a consent token (signed JWT) that references the phone_hash and recipient_id. Store the token jti in an immutable log (append-only or WORM) for compliance auditors.

// Consent webhook payload (example)
{
  "recipient_id": "rec_12345",
  "phone_hash": "uE6...",
  "consent_token": "eyJhbGciOiJSUzI1NiIsInR5cCI6...",
  "consent_ts": "2026-01-15T13:22:00Z"
}
  

Operational playbook: Step-by-step

1. Ingest numbers into a staging queue; do not write raw numbers into long-term stores.
2. Perform initial hygiene: normalize to E.164, remove obvious invalids, and run PSI against suppression lists if needed.
3. Compute phone_hash via HMAC with KMS-managed key and insert mapping into identity index (phone_hash → recipient_id).
4. Trigger verification: prefer RCS challenge if the number is RCS-capable; fallback to OTP for others. Capture the attestation object returned by the carrier or device if available.
5. On successful verification, issue a signed consent token and short-lived delivery token; record events in an append-only audit log.
6. Deliver content via RCS using only the delivery token and the stored attestation; route high-risk content through channels requiring additional authentication.
7. Support revocation: revoke tokens, log events, and propagate blacklists via PSI or hashed suppression lists.

Code pattern: Hash then verify — minimal PII storage

A compact Node + pseudocode sequence to show how a verification flow maps to storage while minimizing PII retention.

// 1) Ingest and normalize (temporary only)
let rawPhone = "+14155552671"; // UNTRUSTED input
let normalized = normalizeE164(rawPhone);

// 2) Compute HMAC (server-held secret via KMS)
let phone_hash = phoneHash(normalized, KMS.getKey('phone-hmac'));

// 3) Check suppression via PSI or hashed lookup
if (suppressionIndex.contains(phone_hash)) {
  return {status: 'suppressed'};
}

// 4) Send RCS verification (capability check + challenge)
let rcsResponse = rcs.sendVerification(normalized, {challenge: 'code'});

// 5) On success, create consent token and store only phone_hash -> recipient_id
db.identityIndex.upsert({phone_hash, recipient_id: 'rec_12345', attestation: rcsResponse.attestation});
  

KPIs and metrics to measure success (and risk reduction)

• Verification success rate — percent of numbers mapped to a verified recipient ID (target > 95% for warm lists).
• PII exposure surface — count of systems storing plaintext phone numbers (target: zero outside ingest tier).
• Deliverability lift for RCS — compare open/click rates vs SMS after implementing attestation+RCS (expect 10–40% higher engagement in many promos).
• Fraud reduction — tracked via chargebacks, account takeovers, or failed KYC attempts (use PYMNTS-style benchmarking; financial firms lose billions to gaps in identity controls).

Compliance checklist (GDPR, CPRA, sector rules)

• Document legal basis for storing identifiers (consent, contract) and retention windows.
• Keep a deletion and revocation workflow that can unlink phone_hash → recipient_id and remove the raw number from staging within a short SLA (e.g., 30 days or less depending on rules).
• Use signed consent tokens and append-only logs for audits; exportable in machine-readable formats for regulators.
• Maintain key rotation and proof-of-provenance for hashing secrets (store key identifiers and rotation history with audit logs).

Advanced strategies and future-facing ideas for 2026 and beyond

As the ecosystem matures, consider these advanced tactics:

• Selective disclosure with verifiable credentials — issue W3C-style verifiable credentials that assert attributes (e.g., "has_active_account") without revealing phone numbers.
• Zero-knowledge proofs (ZKPs) — for very high-sensitivity workflows, advanced ZKP-based PSI implementations can prove membership in a list without revealing either side’s inputs.
• Edge attestation — use device-bound keys for persistent, privacy-preserving session tokens for repeat delivery workflows.
• Cross-channel identity hubs — consolidate identity mapping across RCS, email, and app push via hashed linkages so you can route content to the best channel without storing raw identifiers centrally.

Real-world example (anonymized case study)

A mid-size fintech in 2025 replaced OTP-SMS with an RCS-first verification flow. They implemented HMAC-based phone hashing, carrier attestations for 60% of their audience, and PSI for suppression lists. Results in 6 months:

• Verification success rose from 88% to 96% (because fewer false negatives from carrier attestations).
• Operational PII storage decreased by 92% — only the ingest layer retained raw numbers for 24 hours.
• Fraud-related account takeovers decreased 31% year-over-year.

"We stopped storing plain phone numbers for day-to-day operations; that change alone reduced our exposed surface and simplified audits." — Head of Identity, anonymized fintech

Common pitfalls and how to avoid them

• Storing raw numbers in multiple systems — centralize ingestion and make downstream systems use hashed tokens only.
• Assuming RCS is uniformly E2EE — verify per-carrier and per-client capability; fall back to app-based verification if necessary.
• Key management oversight — rotating HMAC keys without mapping plans breaks your lookup ability; design re-hash or multi-key lookup strategies.

Actionable checklist to implement in the next 90 days

1. Inventory systems that store phone numbers and categorize by retention and purpose.
2. Deploy HMAC-based phone hashing for matching and suppression lists; store key IDs and rotation policy in your KMS.
3. Instrument attestation capture in your verification flow — carrier attestation where possible; device/app attestation otherwise.
4. Issue signed consent tokens and keep an append-only audit log. Expose webhook hooks to downstream systems to enforce real-time revocation.
5. Run deliverability tests comparing RCS vs SMS after implementing attestation-based verification and measure KPIs listed above.

Why this matters now — the business case

Fraud and identity risk costs continue to climb. As a 2026 PYMNTS report highlighted, financial firms and other digital-first businesses systematically underestimate identity risk — costing tens of billions globally. Minimizing PII and using cryptographic linkage techniques not only reduces breach impact but also improves deliverability and opens up richer RCS experiences for verified recipients.

Closing: Build privacy-first recipient linkage for the RCS era

RCS brings a new era of secure, feature-rich messaging. To reap the benefits without increasing risk, technology teams must adopt HMAC/tokenization patterns, capture and persist attestations (not raw PII), and bake consent and revocation into the delivery architecture. These steps reduce attack surface, improve compliance readiness, and increase trust and deliverability.

Actionable next step

If you’re evaluating vendor options or building in-house, run a 30-day pilot that swaps OTP-SMS for an RCS-first verification path for a subset of users, and measure verification success, PII reduction, and fraud signals. Need an implementation blueprint or sample webhook and JWT templates tailored to your stack? Contact our team for a technical workshop and demo of privacy-first recipient linkage models.

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